It's the way you're forming your loaves. Next time, don't roll it out so flat -- you're completely degassing it, which isn't a good thing. Be gentle. Let the loaves rise in the pan until they're about 2/3 of what you want, then use your knuckles to gently press the bread down about 1/2 way to create a channel all the way down the length of the bread, and let the bread sits until it's about 3/4 of what you want. Now you can bake.
You'll like the traditional sandwich loaf with a heart-shaped top the final "punch down" gives.
Good luck,
BDL